Modern integrated circuits (ICs) are easily damaged by excess voltages, Common sources of these potentially damaging voltages include electrical overstress (EOS) and electrostatic discharge (ESD). ESD, a serious issue in solid state electronics, is a transfer of electrostatic charge between bodies or surfaces at different electrostatic potentials either through direct contact or through an induced electrical field. ICs which are built using semiconductors, such as silicon, and insulating materials, such as silicon dioxide, can be permanently damaged when subjected to higher voltages that may be produced by ESD events.
Traditionally, on-chip circuits are employed to protect the IC during an ESD event. In conventional IC ESD protection schemes, special clamp circuits often shunt ESD current between the IC power supply rails and thereby protect sensitive internal elements of the IC from damage. Such clamping circuits have a timer circuit (e.g., a resistor-capacitor (RC) timer, which may be referred to as a “transient detector”) and a large n-channel MOSFET device for discharging the high ESD current. Thus, a power rail clamp circuit is often employed within an IC so that if an ESD event is encountered on the IC's power rail, the clamp will turn on and reduce the voltage so that the IC's main devices (circuitry elements) will not be damaged. Implementations and use of such RC clamps are well-known in the art.
The RC-clamp size is very large and uses most or all of the chip's metal layers to provide low resistance and high current handling capability. Previously, the RC timer and inverter portions of the RC clamp have been located in one pad (in the pad ring) and large field effect transistors (bigfets) have been distributed across the chip's pad-ring. In other previous implementations, the RC clamp has been configured as a very large one piece RC-clamp containing all the building blocks. Each of these designs occupies a large portion of the chip's metal layers and thereby greatly constrains routing options for the chip's other functional components.